1. Technical Field
The present invention relates to a photovoltaic device.
2. Related Art
Photovoltaic devices are known in which an amorphous silicon layer which is substantially intrinsic is formed between a crystalline silicon substrate and a doped amorphous silicon layer.
As a means for improving an output characteristic of the photovoltaic device having such a structure, a structure is known in which an oxygen concentration of an interface portion between the silicon substrate and the intrinsic amorphous silicon layer is increased (refer to Japanese Patent No. 4070483). In addition, a structure is known in which a slope is provided in the oxygen concentration in the intrinsic amorphous silicon layer, with the oxygen concentration of the side of the doped amorphous silicon layer being higher (refer to JP 2008-235400 A).
On the other hand, it has been reported that in an inactivation technique of a surface of a silicon substrate by the intrinsic amorphous silicon layer, by containing oxygen in an appropriate amount in the entirety of the intrinsic amorphous silicon layer and forming a very small amorphous silicon oxide region within the layer, it is possible to promote the inactivation (refer to J. Appl. Phys. 107, 014504 (2010)). In addition, it has been reported that by containing oxygen in an appropriate amount in the entirety of the intrinsic amorphous silicon layer, the output characteristic of the photovoltaic device can be improved (refer to Appl. Phys. Lett. 91, 133508 (2007).
When excessive oxygen is introduced into the amorphous silicon layer, the oxygen may act as an impurity to form a defect or form a high-resistance region. Therefore, optimization of the oxygen concentration to be contained is desired. However, the optimum oxygen concentration profile in the intrinsic amorphous silicon layer, in particular, the oxygen concentration at the side of the interface between the silicon substrate and the intrinsic amorphous silicon layer, has not been sufficiently investigated.